The present invention relates to electrodes for a drop-on demand printer of the type described in WO-A-9311866 and, more particularly in WO-A-9727056, in which an agglomeration or concentration of particles is achieved at an ejection location and, from the ejection location, particles are then ejected onto a substrate for printing purposes. The present invention relates to controlling the resistance of electrodes in the printer in order to prevent electrostatic discharge.
In WO-A-9727056 we describe an apparatus which includes a plurality of ejection locations disposed in a linear array, each ejection location having a corresponding ejection electrode so that the ejection electrodes are disposed in a row defining a plane. One or more secondary (intermediate) electrodes are disposed transverse to the plane of the ejection electrodes in front of the ejection locations so that the sensitivity of the apparatus to influence by external electric fields can be reduced. The sensitivity to variations in the distance between the ejection location and the substrate on to which the particles are ejected is also reduced. The secondary electrode is preferably disposed between the ejection electrodes and the substrate and may comprise a planar electrode containing a central slit through which particles are ejected on to the substrate or plural secondary electrodes.
Electrostatic discharge may occur between the ejection electrodes and the intermediate electrodes, causing misprinting.
In WO 02/05708 we describe how electrostatic breakdown can be prevented by including a resistive element adjacent to an intermediate electrode, on a conductive track which supplies a voltage to the intermediate electrode.
As an alternative to the use of a resistive element adjacent to an intermediate electrode, electrostatic discharge can be prevented by coating the intermediate electrode surface with an insulator. In practice, however, if the resistance of the coating is too large then the surface of the insulator on the electrode charges up due to the build up of leakage current between the electrodes or the electrostatic attraction of naturally occurring charged particles, such as dust. As this charge builds up it opposes the applied field, reducing its strength and therefore compromising the operation of the system that requires a high electric field. It is conceivable that this charging rate may vary over several orders of magnitude (depending on the exact nature of the system) therefore one needs to be able to tune the resistance of the film in order to achieve the correct balance between the protective nature of the coating whilst ensuring that charge does not build up on the surface of the coating. This could be achieved by controlling the thickness of the coating; however, because most insulators have a bulk resistivity of approximately 1014-1015 Ωm the film would have to be impractically thin to achieve the desired resistance using a standard insulator. Thus, in order to be able to achieve the required resistance using a practical film thickness that can be produced as a defect free film, it is necessary to find a material with a resistivity that is lower than this, and which can be tuned to eliminate the effect of the charging mechanisms.
Unfortunately, very few naturally occurring materials exhibit a resistivity in the required range of 102-1014 Ωm. Elemental metals have a resistivity of approximately 10−6-10−7 Ωm and insulators have a resistivity of greater than approximately 1013 Ωm. Semiconductors have resistivities that are dependent on temperature and doping density, to name but two variables, but in this case it is impractical to consider using these variables as a method of tuning the resistivity.
An aim of the present invention is to reduce the likelihood of electrical breakdown and electrostatic discharge between the electrodes.